1. Field of the Invention
The present invention relates generally to a reinforced secondary backing and, more particularly, to carpets that use the reinforced secondary backing and a method of forming a carpet with that reinforced secondary backing.
2. Background
Laminate carpets typically include a secondary backing forming their lower surface and a primary backing tufted with yarns forming their upper surface. Latex is often the adhesive used to bond the primary and secondary surfaces together. That latex, or binder, is usually applied between the primary and secondary backings and then immediately “sandwiched” between these two backings by compressing marriage rollers. The formed carpet next passes through an oven where water is removed from the latex to cure it. The cured latex binds the tufted primary backing to the secondary backing and the formed laminate carpet is then ready to be installed in a residential or commercial setting.
As one skilled in the art appreciates, the secondary backing is stretched apart by pulling its edges away from each other during the processing. In addition there may be some contraction when the secondary backing is exposed to heat in the curing oven during the manufacturing process; however, the opposed edges of the secondary backing are held stationary relative to each other when subject to this heating and associated shrinkage. Thus, although the secondary backing contracts from the heat during the manufacturing process, its width is maintained at a set distance so that the dimensions of the carpet remain substantially constant.
Another problem, however, arises from the secondary backing being stretched, namely, stresses develop in the secondary backing and weaken the final carpet structure. More specifically, the resulting tension from the stretching or pulling of the secondary backing can break mechanical bonds that its yarns initially form with the latex.
The normal method used to measure the adhesion or holding strength between the secondary backing and the primary backing is to determine “delamination strength.” Delamination strength represents the “peel resistance” characteristics of secondary carpet backing or, stated differently, the force required to separate the secondary backing from the rest of the carpet.
Another issue related to carpet durability is the tendency of the yarns to separate from the primary backing, which is the “tuft bind.” That is, tuft bind measures the force necessary to pull the tufted yarn from the laminate carpet. Among other factors, tuft bind is a function of delamination strength. Accordingly, it is desirable for delamination strength to be as high as possible to decrease the likelihood of separation of the secondary backing from the rest of the carpet, increase resistance to tufts being pulled out of the carpet, and enhance dimensional stability of the carpet.
To achieve suitable delamination strength, the secondary backing should be in intimate contact with the tufted primary backing and backstitch during the forming and curing process. In addition, the secondary backing must be firmly bonded with and into the adhesive; otherwise, it will be easier to pull the secondary backing away from the primary backing—and correspondingly easier to pluck yarn from the face of the laminate carpet.
As noted above, however, that secondary backing is pulled tautly during manufacturing and then tries to shrink or contract when it is exposed to heat while its edges are maintained at a constant separation distance from each other. Based on the dynamics of the shrinkage, the stresses are not evenly distributed across the width of the secondary backing; instead, those stresses are concentrated at the edges of the carpet. For example, in a twelve (12) foot wide carpet, approximately eighty percent (80%) of resulting weakness from this shrinkage and associated tension exists in the area from the edge extending inwardly approximately four to twelve (4–12) inches. Thus, as those skilled in the art appreciate, the resultant weakness in the formed carpet is most pronounced near its edges.
To address this problem of low delamination strength adjacent the carpet edges, one attempted solution has been for carpet manufacturers to increase the weight of adhesive applied to the secondary backing. The obvious drawback to this manufacturing modification is the corresponding cost increase of the carpet, as adhesive is an expensive component in laminate carpets. This attempted solution of uniformly increasing adhesive also does not address the differential in strength between the areas adjacent the edges of the carpet and the areas farther inward from the edges.
Another attempted solution to this problem has been for carpet manufacturers to increase the quantity or weight of adhesive applied near the edges of the secondary backing. For example, carpet manufacturers may apply adhesive at a rate of an extra two to four ounces per square yard in the area of the secondary backing spanning from its edges to six to eight inches inward. As noted above, the extra adhesive increases the cost of the laminate carpet. In addition, this additional adhesive applied to the edges may result in uneven curing, as the edges take longer to cure than the interior sections of the carpet.
Therefore, a need exists in the art to increase effectively the delamination strength of carpet, particularly near its edges. A need also exists to be able to increase the delamination strength of carpet at selected areas based on the intended use of the carpet. For example, it may be desired to raise the delamination strength in the center of the carpet if it is anticipated that the carpet will be longitudinally cut at that location before use or installation.